1. Field of Invention
The present invention relates to a driving device. More particularly, the present invention relates to an organic light emitting diode (OLED) driving device.
2. Description of Related Art
FIG. 1 shows a circuit diagram of a conventional OLED driving device. The OLED driving device in FIG. 1 comprises P-type metal oxide semiconductor (MOS) transistors 101˜106 for driving an OLED 107. A control end of the transistor 101 receives a first select signal S1, and controls the input of a data signal Vdata. The data signal Vdata is input into the storage capacitor 104 via the transistor 102, and the voltage on the storage capacitor 104 controls the gate of the transistor 105. Thus, an output end of the transistor 105 generates a current corresponding to the voltage. The gate of the transistor 106 is controlled by a second select signal S2. The current generated by the transistor 105 passes through the transistor 106 and drives the OLED 107 to emit light, and the charges of the storage capacitor 104 are released via the transistor 103.
FIG. 2 shows a voltage-to-time waveform chart of the data signal Vdata, the first select signal S1 and the second select signal S2. During the time period T11˜T12, the first select signal S1 is at a low level, and this is the reset period of the storage capacitor 104. During the time period T12˜T14, the data voltage Vdata is at a high level, and it is the data storing period of the storage capacitor 104. During the time period T13˜T14, the second select signal S2 is at a low level, and it is the light-emitting period. At this time period, the OLED 107 emits light due to the current generated by the transistor 105.
The conventional circuit design is using the transistor 102 to compensate the effect of the threshold voltage of the transistor 105. The threshold voltages of the transistor 102 and the transistor 105 are respectively Vth-102 and Vth-105, the current flowing through the transistor 105 is I, and k is a proportional constant. Thus, the following relation can be obtained:I=(k/2)*(Vdata−|Vth-102|−VDD+|Vth-105|)2 
Under ideal conditions, the threshold voltage Vth-102 of the transistor 102 is identical to the threshold voltage Vth-105 of the transistor 105, and thus the following relation can be obtained:I=(k/2)*(Vdata−VDD)2 
It may be known from this relation that, the current I is not affected by the threshold voltages. However, the disadvantage of the conventional circuit is the mechanism of using the data signal Vdata to discharge the storage capacitor 104. If the voltage at the node n1 is close to that of the data signal Vdata, charges of the storage capacitor 104 cannot be completely released. Moreover, the charging mechanism cannot be adapted for integrated circuits with operating voltages of different specifications.